Katie had shown up to meet Ron and the others as soon as she'd gotten the first patronus - before that she'd been pacing around her apartment wringing her hands, and it felt good to at least have the momentary action of getting to the ministry. She'd made sure her father had left for the night already - there wasn't too much Quidditch research to do at night - and then there had been more pacing, and more waiting. It seemed almost like nothing was going to happen, and then all of a sudden they were everywhere, matching her friends in battle out of nowhere.
It felt like a flashback to Hogwarts, to see people she loved fighting people they all hated. She'd managed to disarm one of the slower cloaked men, and was in the process of of sending stinging hexes at another when she saw George kill Lestrange. Satisfied that the man she was hexing was in a sufficient amount of pain that he wouldn't come after her, she started to work across the room towards George, keeping her eyes open and flinging spells everywhere they were needed.